TRAVEL ADVISORY: CORRESPONDENT'S REPORT; Security Stepped Up At U.S. Airports

AIRLINE passengers may not notice, but Government officials say that steps taken since the beginning of the year have made travel more secure.

On Jan. 1 the airlines began modernizing the system they use to decide which passengers -- and their bags -- deserve closer scrutiny. For 25 years the airlines have relied upon their ticket agents to make that judgment, but this year they began switching to a computer to do the job, and the Federal Aviation Administration has set the end of 1998 for all airlines to be using computers. The system, called Computer Assisted Passenger Screening, relies on secret criteria to determine who might be trying to smuggle a bomb on board; it also includes for scrutiny a random sample of those who would ordinarily not be searched.

Security officials with the airlines and the Government would not say what fraction of passengers will normally be allowed through by the computers under the new system without close attention, but it's likely to be a majority of passengers. For the minority who are ''selected'' there are two changes in how their baggage is handled.

First, their checked baggage (not carry-on) is put through a scanner that measures the shape and density of objects inside, to sense their chemical composition. Such scanners did not exist a few years ago. Engineers are working on more sophisticated machines; one problem with these is that they can leave stripes in undeveloped film in the bags.

If no such equipment is available, and in most American airports it is not, the other method is to make sure that if the bag gets on the plane, the passenger does too, a technique called bag matching. Matching has been in place on international flights for years, but is new to domestic travel.

The five-year plan, beginning last October, is for more scanning and less matching. Vice President Gore, in a speech at San Francisco International Airport on Jan. 30, said the solution lay in ''tapping the awesome power of our newest technology to shield us from the hazards of a sometimes hostile world.'' But the cost is awesome, too, with the bag scanners running from $700,000 to $1 million each.

The F.A.A. reported at the end of January that it had purchased 54 explosive-detection systems and 22 automated X-ray devices, both for scanning checked bags. The budget submitted by President Clinton in January calls for $100 million for the purchase of 88 more scanning devices, to be spread among airports around the country, on top of a $144 million budget this year. This still would not be enough for one high-tech scanner per airport, and many airports would need many more than one.

At Vivid Technologies, of Woburn, Mass., Jeff Agnew, a spokesman, said that ''Europe, and Asia following it, is trying to rapidly pursue 100 percent scanning. The U.S. is a long way from that.'' Most major European airports have several such devices, as do many big airports in Asia.

Vivid is supplying a system that will screen every bag at the new Terminal One at Kennedy Airport, where Japan Air Lines, Air France, British Airways and Korean Air will operate. The first airlines are set to begin service there on May 22. Vivid and two competitors, L-3 Communications of New York and Invision Technologies of Newark, Calif., expect to have more advanced machines ready for testing by the F.A.A. this summer. Exactly what the machines do -- and how small a quantity of what explosive they can find -- is secret.

In addition to scanners, the F.A.A. said it will buy 489 units that detect minuscule traces of explosives, which will be used to sniff passengers and objects they have handled, like tickets.

In some airports the explosives scanners sit on public concourses; at others they are in baggage-handling areas, and passengers are probably not aware of them. Generally matching is more obvious, because flights are delayed while baggage handlers climb through the hold to find a bag belonging to a passenger who didn't board the plane.

In addition, the F.A.A. will pay for test models of baggage containers that can withstand a luggage bomb. The airlines have resisted such containers, not only because they are more expensive than the standard aluminum models used on wide-body jets but also because some models are either heavier than aluminum or will not stand up to everyday wear-and-tear as well as aluminum does. The Government is also increasing the number of dogs that are trained to smell explosives; by the end of this year, there will be 130 teams at 38 airports, up from 86 teams at 30 airports in 1996.

The airlines still look on all these efforts with some disdain. They point out that the impetus was the belief, now mostly discredited, that TWA Flight 800 was brought down by a bomb when it exploded in July 1996. That crash led to the appointment of a White House commission on civil aviation, led by Mr. Gore, which recommended changes, including security improvements.

At a recent industry meeting on setting priorities for air safety, one executive said that there was no choice now except to implement new security measures, even if other problems were more important, like navigation errors that send airplanes into mountains.

Correction: April 12, 1998, Sunday The Correspondent's Report article on March 29, about stepped-up security at United States airports, included one airline erroneously among those operating at Terminal One at Kennedy Airport, and omitted another. Lufthansa operates at that terminal; British Airways does not.